1) Bit of clique humor 2) Gag meant for a select few 3) It may be hard to get 4) Private gag 5) Private humor 
An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup, that is, people who are in a particular social group, occupation, or other community of shared interest. It is an esoteric joke that is humorous only to those who are aware of the circumstances behind it. In-jokes may...
[n] - a joke that is appreciated only by members of some particular group of people
noun a joke that is appreciated only by members of some particular group of people
a joke that can be understood or appreciated only by the members of a limited group of people.